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Once a pet lizard or human (depending on the source material) who was covered with mutagen, Mondo Gecko was transformed into a half human hybrid and raised by a criminal until he decided to fight for good.

Mondo Gecko was once a normal gecko who was in the sewers and was exposed to the same mutagen that transformed four turtles into the Teenage Mutant Ninja Turtles. Mondo Gecko was soon found and picked up by the mysterious criminal known only as Mr. X, who raised him to become his own personal thief. Years later, the Ninja Turtle Michaelangelo had dreams where he remembered the existence of Mondo and decides to investigate. When he met Mondo Gecko, he learned he was being used by Mr. X and convinced him to turn on his criminal father figure and fight for good.

From the 1987 animated series

Later, the Turtles recruited the help of Mondo when mutants all over New York City where being hunted by a man named Dirk Savage. Together they were able to defeat Dirk and free the mutants from captivity.